Parents know that crying babies usually calm down when they are picked up and carried, and a new study explains why.

Researchers at the RIKEN Brain Science Institute in Japan demonstrated that the infant-calming response is prompted by the baby's sense of movement and touch. Using tiny electro-cardiogram systems, the scientists showed that being carried about led to an immediate slowing of the infant's heart rate. This was not the case when infants were simply held.

They observed the phenomenon in human and mouse babies, leading Kumi Kuroda, who led the study team, to suggest the response may be an essential part of the mother-infant bonding process.

"It reduces the maternal burden of carrying and is beneficial for both the mother and the infant," she says.

The findings, published in the journal Current Biology, have implications for parenting and could help prevent child abuse. Kuroda notes that the unnerving effect on adult caregivers of an infant's unsoothable crying is a major risk factor for child abuse. If parents understand why their baby is crying and how to respond to it, they might be less frustrated.